1. Field of the Invention
The present invention relates to a triple effect absorption refrigeration system. Particularly, the present invention relates to a triple-effect absorption refrigeration system using a direct-fired heat source to heat its high temperature generator. More particularly, the present invention relates to the recovery of energy from the combustion products generated by the direct-fired heat source and the utilization of this energy within the absorption cycle of a triple-effect absorption refrigeration system.
2. Description of the Related Art
Absorption refrigeration systems are generally used to cool commercial buildings. A single-effect absorption system typically comprises a generator, a condenser, an evaporator and an absorber. In this system, a refrigerant-containing absorption solution is heated in the generator by an outside heat source, such as a fuel burner, low-pressure steam, or hot water, in order to boil off refrigerant vapor. The refrigerant vapor is condensed to refrigerant liquid, and then routed to an evaporator. The refrigerant liquid in the evaporator absorbs the heat from the air in the commercial building being cooled, again flashing to vapor. The refrigerant vapor flows to an absorber, where it mixes with an absorption liquid, and the combined refrigerant-containing absorption solution is pumped to the generator.
The single-effect absorption system described above is extremely inefficient, having a thermal Coefficient of Performance (COP) of approximately 0.7.
A more modem approach is to use a double-effect absorption refrigeration system. In this latter system, the single generator and condenser are replaced by two generators (a high temperature generator and a low temperature generator) and two condensers (also high temperature and low temperature). Primary heat is supplied to the high temperature generator to boil off refrigerant vapor from the refrigerant-containing absorption solution. The refrigerant vapor is condensed in the high temperature condenser. The heat of condensation from the high temperature condenser is used to heat the refrigerant-containing absorption solution in the low temperature generator, boiling off more vapor in that generator. In this manner, the heat input to the system is utilized twice to generate refrigerant vapor. The thermal COP of a double-effect absorption system hence is improved to approximately 1.2.
In recent years, experiments have been conducted with triple-effect absorption systems, utilizing three generators and three condensers with a single absorber and a single evaporator. Triple-effect absorption systems use the primary heat input to the high temperature generator three times to generate refrigerant vapor. Various configurations of known related triple-effect absorption systems are discussed below.
FIG. 1 depicts one known triple-effect absorption system, in which an absorber A provides refrigerant-containing absorption solution to three generators, including a high temperature generator G3, an intermediate temperature generator G2, and a low temperature generator G1. In this system, the absorption solution is supplied to the three generators from absorber A via a parallel flowpath connection. Each generator feeds refrigerant vapor to a corresponding condenser, including a high temperature condenser C3, an intermediate temperature condenser C2, and a low temperature condenser C1. Furthermore, the higher temperature condensers C3 and C2 are coupled with the lower temperature generators G2 and G1, respectively. Hence, the system is referred to as a double-coupled condenser (DCC) triple-effect absorption system. Heat exchangers HX1, HX2, and HX3 can be provided in the parallel flowpath from the absorber. The thermal COP for this parallel solution flowpath system is calculated to be 1.730.
FIG. 2 depicts another known DCC triple-effect absorption system. In this configuration, generators G1, G2 and G3 are connected in an inverse series flowpath connection with absorber A, rather than in a parallel flowpath connection as in FIG. 1. Heat exchangers HX1, HX2 and HX3 are provided in the inverse series flowpath.
FIG. 3 depicts yet another known DCC triple-effect absorption system, with the generators G3, G2 and G1 connected to the absorber A in a series flow arrangement. Heat exchangers HX1, HX2 and HX3 are provided in the series flowpath. The thermal COP for this series solution flowpath system is calculated to be 1.608.
FIG. 4 depicts still another known DCC triple-effect absorption system, having an inverse parallel series solution feeding arrangement, which system is fully described in U.S. patent application Ser. No. 08/743,373, filed Nov. 4, 1996, and entitled Triple Effect Absorption Refrigeration System, the disclosure of which is incorporated herein by reference. Generators G1 and G2 are connected to absorber A in a parallel flow arrangement and generator G3 is connected to generator G2 in a series flow arrangement. A weak solution, which means a solution containing a low ratio of absorption fluid to refrigerant, is fed from the absorber into the low temperature generator G1 and the intermediate temperature generator G2 in parallel. The solution in G2 is heated, and refrigerant vapor is boiled off. The now more concentrated solution, containing a higher ratio of absorption fluid to refrigerant, is sent from G2 to high temperature generator G3. The solution is further concentrated as more refrigerant is boiled off, and then exits G3. Likewise, the weak solution in G1 is concentrated as refrigerant is boiled off, and the more concentrated solution exits G1. This solution exiting G1 mixes with the more concentrated absorption liquid exiting G3, and returns to the absorber. The system primary energy is input to G3, where it heats the solution and generates refrigerant vapor as described above. The refrigerant vapor generated from G3 is condensed in high temperature condenser C3, and the heat of condensation is exchanged with G2 in order to generate refrigerant vapor in G2, as described above. The condensate from C3 and the vapor from G2 pass intermediate temperature condenser C2. The heat of condensation in C2 is exchanged with G1 in order to generate refrigerant vapor from G1 as described above. The condensate from C2 and the vapor from G1 collect in low temperature condenser C1, and the resulting condensate is sent to evaporator E in order to obtain the desired refrigeration effect. The resultant low pressure vapor is then passed from the evaporator to the absorber, where it combines with the returning concentrated solution to dilute the solution and begin a new cycle. Computer simulations show that the inverse parallel series solution feeding arrangement provides the highest thermal COP (1.736) of all the aforementioned DCC triple-effect absorption systems.
All of the various DCC triple-effect absorption systems use a primary energy source to supply heat to the high temperature generator for boiling off refrigerant vapor from the refrigerant-containing absorption solution. One such energy source is a direct-fired apparatus, which generates products of combustion or flue gases. After transferring heat to the solution, the combustion gases are discharged from the generator via a chimney or flue. Because the combustion gases are discharged at an elevated temperature, there is energy lost in the discharge of these gases.
The use of heat recovery systems to recoup the energy from flue gases in single effect and double effect absorption refrigeration systems is limited by practical considerations. First, there exists a tradeoff between the value of the energy that can be recovered from the flue gases and the cost of the additional heat exchanger surface necessary to recover that energy. Second, excessively low flue gas temperatures can result in condensation of corrosive gases, particularly if sulfur is present in the fuel. This can lead to undesirable corrosion in the generator or flue. Thus, for any given flue gas temperature, the energy that can be recovered is limited by the condensation temperature of these corrosive gases. Improvement in combustion efficiency through recuperation of the energy in flue gases is restricted by these factors, and as a result, is generally impracticable for single and double effect systems.
However, in order to achieve high performance levels, triple effect absorption cycles operate at higher temperatures than double effect or single effect cycles. Typically, triple effect cycles are calculated to operate with solution temperatures in the high temperature generator that are 100.degree. F. to 150.degree. F. higher than in a double effect cycle. For example, typical double effect refrigeration systems, using a lithium bromidewater solution, operate with solution temperatures leaving the high temperature generator of 300.degree. F. to 330.degree. F., while analysis of similar triple effect refrigeration systems shows they would operate with solution temperatures leaving the high temperature generator of 400.degree. F. to 480.degree. F.
The focus in the design of triple effect absorption refrigeration systems has been to try to limit the maximum solution temperatures in the high temperature generators. High solution temperatures create corrosion problems within the solution's flowpath. Furthermore, reducing the solution temperatures in the high temperature generators would result in a reduction in the temperature of the exhaust gas exiting from the generator. Lower exiting exhaust gas temperatures mean an increase in combustion efficiency.
Thus, prior experience with single and double effect absorption refrigeration systems biases one away from recouping the energy from the direct-fired burner's combustion products. And the trend in the triple effect absorption refrigeration system industry has been to reduce both the solution temperatures in the high temperature generators and the corresponding temperatures of the exhaust gas exiting from the high temperature generator.